MODELING UNDERTOW DISTRIBUTIONS UNDER LOW-WAVE CONDITIONS USING XBEACH

Abstract

 In nearshore morphodynamic modeling, the simulation of offshore-directed mean current or so-called undertow is necessary for sediment transport as a significant driver. XBeach model is wildly used in predicting storm impacts on natural coasts, yet in regard to morphodynamic predictions, the undertow simulations have been rarely discussed. Recent improvements in the XBeach undertow prediction have been made by modifying the wave roller model. However, despite the improvements for high-wave conditions, the XBeach still performs poorly in predicting undertow under low-wave conditions, where the water depth is a dominant factor controlling undertow current instead of wave mass flux. To improve the XBeach undertow prediction, this paper proposes a modification based on the water depth effect. XBeach performance was tested by comparing two-dimensional simulations with the field observations during low-wave period conditions at Hasaki coast, Japan.

 Calibration of model coefficients yielded good results for waves, though low accuracy for the simulated undertow. Here, the individual contributions of Stokes drift and Lagrangian velocity (wave forcing term) were respectively modified by including a water depth coefficient. This adjustment caused the undertow’s water depth to be considered from under the wave trough level. The temporal undertow comparisons show a small improvement in the whole period and the accuracy correlates with tidal elevation. When the comparisons of undertow spatial distribution were considered at the different water levels, the undertow results were significantly improved especially at low tide, with an increase in accuracy from bad to fair.